Depressive symptoms are prevalent among pregnant women and consistently linked with adverse outcomes for both women and infants, including higher rates of spontaneous abortion, preeclampsia, operative delivery, and postpartum depression. Infants of depressed pregnant women are more likely to be born pre-term, and experience more language and cognitive delays, anxiety disorders, and attention deficit / hyperactivity disorder. In spite of these risks, few interventions have been developed to reduce prenatal depressive symptoms. Because pregnant women are often reluctant to take antidepressants, the most available form of care, a pressing need exists to evaluate interventions that are efficacious in reducing symptoms and more acceptable and accessible to pregnant women. Increased physical activity has numerous advantages as a strategy to decrease symptoms of depression. Moderate intensity exercise is effective in lowering depressive symptom levels among individuals who are not pregnant. It is also inexpensive, safe, and associated with multiple positive health outcomes. The American College of Obstetricians and Gynecologists strongly recommends regular physical activity throughout pregnancy, yet, in practice, many pregnant women are unsure of how to safely adhere to this recommendation. Findings from our research indicate that a tailored, supported physical activity intervention would be acceptable to depressed pregnant women and would be preferable over pharmacotherapy. Plausible mechanisms exist by which increased physical activity may reduce depressive symptoms, including physiological, behavioral and psychological factors. No study to date, however, has evaluated physical activity as an intervention for depressed pregnant women. Our team recently developed a gentle, 10-week, pedometer-based walking intervention designed for pregnant women, the Prenatal Walking Program (PWP), including detailed intervention manuals, interventionist training programs, and adherence scales. The program is designed to be low-cost and transportable for delivery in community settings. We evaluated PWP in an open trial and found evidence for the feasibility, acceptability, and safety of the intervention. In addition, participants reported significant reductions in depressive symptoms and functional improvements. The research plan for this R01 application includes a fully-powered RCT to evaluate PWP in comparison with a perinatal-focused Health Education Control (HEC) condition. 152 pregnant women will be randomized to PWP or HEC and will complete blind assessments at multiple points across pregnancy and postpartum. The primary aim is to examine whether the PWP group has greater reductions in depressive symptoms relative to HEC. In addition, several key maternal health and functioning outcomes will be assessed, and infant neurobehavioral exams will allow for examination of group differences. Potential mechanisms will also be tested, including behavioral factors (behavioral activation, decreased avoidance), psychological factors (increased self-efficacy), and physiological factors (decreased inflammation, improved sleep).